Magnetic Specific Heat of Anion-Radical Salt [Ni(bipy)3](TCNQ)4·(CH3)2CO at Very Low Temperatures

D. Šoltésováa, E. Čižmár a, G. Vasylets b, V.A. Starodub c and A. Feher a aInstitute of Physics, Faculty of Sciences, P.J. Šafárik University, Park Angelinum 9, 041 54 Košice, Slovakia bApplied Chemistry Department, V.N. Karazin Kharkiv National University, Svobody Sq. 4, 61022 Kharkiv, Ukraine cInstitute of Chemistry, Jan Kochanowski University of Humanities and Sciences, Świętokrzyska 15G, 25-406 Kielce, Poland

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Thermodynamic studies of the anion-radical salt system [Ni(bipy)3](TCNQ)4·(CH3)2CO, where TCNQ is 7,7',8,8'-tetracyano-quinodimethane, are reported. The anion-radical salt systems based on TCNQ belong to a material class in which the arrangement of the anion-radical salt has considerable impact on the charge transfer and magnetic properties. The crystal structure of the studied compound consists of [Ni(bipy)3]2+ cations containing Ni2+ ions and four types of crystallographically independent anion-radicals TCNQ·- (A, B, C and D). These TCNQ·- radicals form: two different types of TCNQ·- stacks (AABB and CCDD), where a strong exchange interaction is expected. The temperature dependence of the specific heat of a single crystal was studied in magnetic fields up to 5 T and in the temperature range from 0.4 K to 30 K. The temperature dependence of specific heat displays a broad Schottky-like maximum above 0.4 K. Using a single-ion approximation, the analysis of the temperature dependence of the specific heat below 10 K yields values for the anisotropy parameters, D/kB=-1.95 K and E/kB=0.3 K. These results suggest that the observed maximum in the specific heat originates from Ni2+ ions while the exchange interaction between the transition metal ions and the TCNQ is negligible.

DOI: 10.12693/APhysPolA.131.925 PACS numbers: 65.40.Ba, 75.30.Et, 75.30.Gw